The present invention relates to semiconductor wafer manufacturing and, more specifically, to a wafer handling system and method for transporting wafers during wafer processing.
During semiconductor fabrication, ingots are formed and sliced into wafers. The wafers are processed in many steps that require the wafers to be temporarily stored, transported and positioned. Wafer cassettes are often used for temporary storage and transportation of wafers. Cassettes are typically slotted containers which are open at the top and which have four sidewalls and a bottom. Inner surfaces of two opposing sidewalls are fluted to create slots. Individual wafers are each positioned within a respective slot and are held therein, separated from adjacent wafers. When a cassette is fully loaded, the wafers are located in respective slots and spaced apart from one another so that wafers do not abut.
Wafer containing cassettes are typically delivered to a wafer loading station. At such a station, wafers from the cassettes are loaded, for example onto a conveyor. The conveyor transports the individual wafers to a processing station where a manufacturing step takes place. Following the processing step or steps, the wafers are unloaded from the conveyor and delivered into a cassette at a wafer unloading station.
The loading station typically includes a mechanism for removing individual wafers from the cassette and transferring them to the conveyor. The unloading station also typically includes an unloader for returning the individual wafers to cassettes.
In one known approach, wafers are transferred from a conveyor to waiting cassettes utilizing a water slide unloader which, during operation, provides a continuous sheet of water across the entire slide, regardless of whether a wafer is present on the slide. The slide is tilted downwardly at an angle relative to horizontal so that water will flow down the slide. The water slide of this known construction has a uniformly flat planar bottom surface. As a wafer enters the top of the slide, it is supported entirely from below by the water sheet. Therefore, the undersurface of the wafer, adjacent to the upper surface of the slide, does not come into contact with the slide. Any such contact could scratch or otherwise damage the partially processed wafer. As the wafer travels down the slide, it in effect surfs on the sheet of water. That is, the leading (downwardmost) edge of the wafer is elevated relative to the trailing edge by the water as it travels down the slide. In water slides of this type known to the inventors, substantial quantities of water are caused to flow down the water slide. For example, it is not unusual for these known water slide systems to use one hundred gallons of water per hour or more.
These water intensive systems are disadvantageous because relatively expensive distilled water is typically used in these applications. Moreover, water is becoming a scarcer and more costly resource.
Among the known loaders which remove wafers from the cassette are mechanisms described in U.S. Pat. No. 5,299,901. This latter patent specifically describes mechanisms for removing wafers from cassettes and transferring them to a wafer "boat". Early mechanisms used mechanical tweezers or vacuum tweezers to grip individual wafers and remove them from the cassette and move them. However, these gripper mechanisms could damage some of the wafers. A later transfer mechanism used a "pusher" to push individual wafers up and out of a cassette. A pair of clamps would then grip the wafer and remove it.
In the wafer loader described in the '901 patent, a cassette is attached to a platform and inverted. A wafer boat is located below the inverted cassette. A "lifter" is raised into position below a wafer so that the wafer comes to rest on the lifter. The lifter is then lowered and the wafer, resting on the lifter, descends into the awaiting boat. The wafer is thus loaded from the cassette to the boat.
The wafer transfer mechanism described in U.S. Pat. No. 5,299,901 thereby transfers wafers between devices that can be moved and located vertically atop one another. It does not appear capable of transferring a wafer to another device that is located in a horizontal plane; nor does the transfer mechanism of this patent appear capable of placing a wafer on a horizontal surface. These are some of the limitations that prevent this prior art transfer mechanism from efficiently transferring wafers to some types of work stations and horizontal conveyors.
Another known wafer loader has an elongated arm which is inserted beneath a wafer in a cassette. The arm is removed from the cassette with a wafer coupled to the arm using a vacuum. The arm is then rotated about its longitudinal axis to turn the wafer over for delivery to a second location.
U.S. Pat. No. 5,525,024 discloses yet another form of cassette loader.
Although these prior art wafer handling and transfer mechanisms exist, a need nevertheless remains for an improved wafer handling system and method.